The present invention relates to a chuck for gripping workpieces and, more particularly, to a chuck having limited pull-back when gripping workpieces thereby, reducing forces that tend to distort the workpiece.
Draw-down chucks are widely used to grip workpieces while machining or performing other operations. Draw-down chucks come in a variety of sizes to grip various sized workpieces and typically have two to four jaws that move axially between an extended and a retracted position. To extend or retract the jaws, a puller coupled to the jaw rod moves the jaws axially to grip or release the outer or inner surface of a workpiece. Generally the body of the chuck includes bores through which the jaw rods extend. As the puller moves the jaw rods between the extended and retracted positions, the jaws rods are moved within the bores in the stationary chuck body. It is difficult to grip some workpieces with conventional chucks in a consistent and reliable manner without distorting the workpiece. For example, if the puller is retracted too far, the jaw rods engage against the outer radial surface of the workpiece and may exert too much force, thereby distorting the workpiece. Distortion may also occur from the retaining force of the engaged jaws during the machining process. Some manufacturers use elaborate methods to limit the amount of force exerted by the jaw rods or the location of the puller, but it is still difficult to consistently grip a workpiece with a limited force or limited movement of the jaw rods.